12.5 Neuromusculoskeletal Anatomy, Dermatomes, and Neuropathies

12.5 Neuromusculoskeletal Anatomy, Dermatomes, and Neuropathies

Quick Answer: Neuromusculoskeletal diagnostics require an exact mapping of the central neuraxis, spinal nerve roots, sensory dermatomes, motor myotomes, and deep tendon reflexes. On the NCCAOM examination, radiculopathy questions require pinpointing the specific spinal nerve root: C5 governs shoulder abduction and the biceps reflex; C6 governs wrist extension, the brachioradialis reflex, and thumb/index sensation; C7 governs triceps extension, the triceps reflex, and middle finger sensation; and C8 governs finger flexion. In the lower extremity, L4 governs foot dorsiflexion (heel walking), patellar reflex, and medial leg sensation; L5 governs great toe extension (extensor hallucis longus) and foot dorsum sensation without a reliable reflex; and S1 governs plantar flexion (toe walking), the Achilles reflex, and lateral foot sensation. Among peripheral entrapments, carpal tunnel syndrome affects the median nerve but spares the thenar palm (supplied by the palmar cutaneous branch passing superficial to the retinaculum); cubital tunnel syndrome compresses the ulnar nerve at the medial elbow producing a claw hand and Froment's sign; radial nerve palsy causes wrist drop; meralgia paresthetica produces pure sensory burning over the anterolateral thigh; and common peroneal neuropathy causes foot drop with preserved hip abduction. Crucially, Bell's palsy causes complete ipsilateral facial paralysis including the forehead, whereas central stroke causes contralateral lower facial droop with forehead sparing—a mandatory emergency red flag.

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Spinal Anatomy, Nerve Root Dynamics, and Neurological Levels

Radiculopathy occurs when a spinal nerve root is mechanically compressed or inflamed by an intervertebral disc herniation, osteophytic spondylosis, or spinal canal stenosis. Differentiating the neurological level requires systematic evaluation of the motor myotome, sensory dermatome, and deep tendon reflex arc.

Cervical Spine Anatomy and Nerve Roots

There are 7 cervical vertebrae but 8 cervical spinal nerves:

• Cervical nerves C1 through C7 exit ABOVE their correspondingly numbered vertebrae (e.g., the C6 nerve root exits through the neural foramen between C5 and C6 vertebrae).
• The C8 nerve root exits BETWEEN C7 and T1 vertebrae.
• Thoracic, lumbar, and sacral spinal nerves all exit BELOW their correspondingly numbered vertebrae (e.g., L4 exits below L4).

+-----------------------------------------------------------------------------+
|                   CERVICAL SPINE RADICULOPATHY MATRIX                       |
+-----------------------------------------------------------------------------+
| Level | Primary Motor Myotome       | Deep Tendon Reflex  | Sensory Dermatome|
+-------+-----------------------------+---------------------+------------------+
|  C5   | Deltoid (shoulder abduction)| Biceps Reflex       | Lateral shoulder |
|       | Biceps (elbow flexion)      | (Musculocutaneous)  | & lateral arm    |
+-------+-----------------------------+---------------------+------------------+
|  C6   | Wrist Extensors (cock-up)   | Brachioradialis     | Lateral forearm, |
|       | Biceps (elbow flexion)      | Reflex (Radial)     | thumb & index    |
+-------+-----------------------------+---------------------+------------------+
|  C7   | Triceps (elbow extension)   | Triceps Reflex      | Middle finger    |
|       | Wrist Flexors & Finger Ext  | (Radial)            | & central palm   |
+-------+-----------------------------+---------------------+------------------+
|  C8   | Finger Flexors (hand grasp) | None reliable       | Medial forearm,  |
|       | Hand Intrinsics (lumbricals)|                     | ring & pinky     |
+-------+-----------------------------+---------------------+------------------+
|  T1   | Hand Intrinsics             | None reliable       | Medial upper arm |
|       | (Interossei: abduct/adduct) |                     | & axillary border|
+-----------------------------------------------------------------------------+

Detailed Cervical Level Breakdown:

1. C5 Neurological Level:
   • Motor: Deltoid (innervated via axillary nerve; tests shoulder abduction from 15 to 90 degrees) and Biceps brachii (musculocutaneous nerve; tests elbow flexion).
   • Reflex: Biceps reflex (percussing the biceps tendon in the antecubital fossa).
   • Sensory: Lateral aspect of the arm from the acromion down to the lateral cubital crease.
2. C6 Neurological Level:
   • Motor: Wrist Extensors (extensor carpi radialis longus and brevis, innervated via radial nerve; tested by instructing the patient to make a fist and extend [cock up] the wrist against downward examiner pressure) and Biceps brachii.
   • Reflex: Brachioradialis reflex (percussing the styloid process of the radius).
   • Sensory: Lateral forearm, radial side of the hand, thumb (1st digit), and index finger (2nd digit).
3. C7 Neurological Level:
   • Motor: Triceps brachii (radial nerve; tested by having the patient push outward to extend the elbow against resistance), Wrist Flexors (flexor carpi radialis, median nerve), and Finger Extensors (extensor digitorum, radial nerve).
   • Reflex: Triceps reflex (percussing the triceps tendon directly superior to the olecranon process).
   • Sensory: Middle finger (3rd digit), central palm, and dorsal center of the hand.
4. C8 Neurological Level:
   • Motor: Finger Flexors (flexor digitorum superficialis and profundus, median and ulnar nerves; tested by instructing the patient to interlock and squeeze the examiner's fingers in a tight grasp).
   • Reflex: No clinically reliable reflex.
   • Sensory: Medial distal forearm, ulnar border of the hand, ring finger (4th digit), and little finger (5th digit).
5. T1 Neurological Level:
   • Motor: Hand Intrinsics (Dorsal interossei [DAB: Dorsals ABduct] and Palmar interossei [PAD: Palmars ADduct], ulnar nerve; tested by having the patient abduct their fingers against resistance, or hold a sheet of paper tightly between extended fingers).
   • Reflex: No reliable reflex.
   • Sensory: Medial proximal upper arm and axillary margin.

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Lumbar and Sacral Spine Anatomy and Neurological Levels

In the lumbar spine, intervertebral disc herniations typically occur in a posterolateral direction. Because of the anatomical configuration of the lumbar thecal sac and exit pathways:

• A posterolateral disc herniation at the L4–L5 level classically compresses the traversing L5 nerve root (rather than the exiting L4 nerve root).
• A posterolateral disc herniation at the L5–S1 level classically compresses the traversing S1 nerve root.

+-----------------------------------------------------------------------------+
|                  LUMBOSACRAL SPINE RADICULOPATHY MATRIX                     |
+-----------------------------------------------------------------------------+
| Level | Primary Motor Myotome       | Deep Tendon Reflex  | Sensory Dermatome|
+-------+-----------------------------+---------------------+------------------+
|  L4   | Tibialis Anterior           | Patellar Tendon     | Medial lower leg,|
|       | (Foot dorsiflexion/inversion| Reflex (L2, L3, L4) | medial malleolus,|
|       | -> HEEL WALKING TEST)       |                     | medial foot arch |
+-------+-----------------------------+---------------------+------------------+
|  L5   | Extensor Hallucis Longus    | None reliable       | Anterolateral leg|
|       | (Great toe extension) &     | (Occasionally medial| dorsum of foot,  |
|       | Gluteus Medius (abduction)  | hamstring reflex)   | 1st web space    |
+-------+-----------------------------+---------------------+------------------+
|  S1   | Gastrocnemius & Soleus      | Achilles Tendon     | Lateral foot,    |
|       | (Ankle plantar flexion      | Reflex (S1, S2)     | sole of foot,    |
|       | -> TOE WALKING TEST)        |                     | pinky toe (5th)  |
+-----------------------------------------------------------------------------+

Detailed Lumbosacral Level Breakdown:

1. L4 Neurological Level:
   • Motor: Tibialis anterior (deep fibular nerve; drives ankle dorsiflexion and foot inversion). Functional exam: Instruct the patient to walk on their heels across the examination room. Weakness indicates an L4 motor deficit.
   • Reflex: Patellar tendon reflex (knee jerk), mediated through the femoral nerve.
   • Sensory: Medial aspect of the lower leg, passing over the medial malleolus to the medial longitudinal arch of the foot.
2. L5 Neurological Level:
   • Motor: Extensor hallucis longus (EHL) (deep fibular nerve; drives great toe extension / dorsiflexion against downward examiner resistance), Extensor digitorum longus, and Gluteus medius (superior gluteal nerve; drives hip abduction). Functional exam: Inability to dorsiflex the great toe. Weakness of the gluteus medius produces a Trendelenburg gait (pelvis drops on the contralateral side when standing on the affected leg).
   • Reflex: No reliable deep tendon reflex (the medial hamstring reflex can be tested, but lacks high clinical reliability).
   • Sensory: Anterolateral lower leg, crossing the anterior ankle onto the dorsum of the foot, and classically terminating in the web space between the 1st (great) and 2nd toes.
3. S1 Neurological Level:
   • Motor: Gastrocnemius and Soleus (tibial nerve; drives ankle plantar flexion), Peroneus longus and brevis (superficial fibular nerve; drives foot eversion), and Gluteus maximus (inferior gluteal nerve; hip extension). Functional exam: Instruct the patient to walk on their toes across the room or perform single-leg heel raises. Inability to raise up onto the ball of the foot signifies S1 motor failure.
   • Reflex: Achilles tendon reflex (ankle jerk), mediated through the tibial nerve.
   • Sensory: Lateral border of the lower calf, behind the lateral malleolus, along the lateral aspect of the foot, sole of the foot, and 5th (little) toe.

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Peripheral Nerve Entrapment Neuropathies

Peripheral nerve entrapments produce focal motor and sensory deficits restricted to the specific anatomical distribution of a single peripheral nerve trunk, contrasting with the multi-dermatomal or root-level patterns of spinal radiculopathies.

+-----------------------------------------------------------------------------+
|                   PERIPHERAL NERVE ENTRAPMENTS MATRIX                       |
+-----------------------------------------------------------------------------+
| Nerve           | Entrapment Site       | Sensory Deficit      | Motor Deficit / Signs|
+-----------------+-----------------------+----------------------+----------------------+
| **Median**      | Carpal tunnel         | Volar thumb, index,  | Thenar atrophy, ape  |
|                 | (wrist flexor retin.) | middle, 1/2 ring     | hand, +Phalen/+Tinel |
| **Median**      | Pronator teres        | Above PLUS thenar    | Forearm/pronator     |
|                 | (proximal forearm)    | eminence palm        | pain; resists pronate|
| **Ulnar**       | Cubital tunnel        | Medial forearm, 5th  | Hypothenar atrophy,  |
|                 | (medial epicondyle)   | digit, 1/2 ring      | claw hand, +Froment  |
| **Radial**      | Spiral groove humerus | Dorsal radial hand & | WRIST DROP, finger   |
|                 | ("Saturday night")    | dorsal thumb web     | extension paralysis  |
| **Lat Fem Cut** | Inguinal ligament     | Anterolateral thigh  | NO MOTOR DEFICIT     |
|                 | (ASIS entrapment)     | (burning numbness)   | (pure sensory nerve) |
| **Common Fib**  | Fibular neck / head   | Dorsum of foot &     | FOOT DROP, steppage  |
| (Peroneal)      |                       | lateral lower leg    | gait; spares hip abd |
+-----------------------------------------------------------------------------+

Median Nerve Entrapments: Carpal Tunnel vs. Pronator Teres

1. Carpal Tunnel Syndrome (CTS)

• Anatomy: The carpal tunnel is an inelastic fibro-osseous conduit bounded deeply by the carpal bones and superficially by the transverse carpal ligament (flexor retinaculum). It accommodates 10 anatomical structures: the median nerve and 9 flexor tendons (4 tendons of flexor digitorum superficialis, 4 of flexor digitorum profundus, and 1 of flexor pollicis longus).
• Sensory Symptoms: Numbness, tingling, burning, and nocturnal paresthesias in the palmar distribution of the thumb, index finger, middle finger, and radial half of the ring finger. Patients frequently wake up at night and vigorously shake or flick their hands for relief (the "flick sign").
• Motor Deficit: Denervation atrophy and flattening of the thenar eminence (abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis), producing the "ape hand" deformity and weakness of thumb opposition/abduction (difficulty pinching or buttoning shirts).
• The Crucial Anatomical Rule of Spared Palmar Sensation:
  • In carpal tunnel syndrome, sensation over the base of the thenar eminence (the palm) is completely spared and normal!
  • Reason: The palmar cutaneous branch of the median nerve branches off approximately 5 cm proximal to the wrist crease and courses superficial to the flexor retinaculum, completely bypassing the carpal tunnel. Finding numbness over the central palm rules out isolated carpal tunnel syndrome.
• Provocative Examination Tests:
  • Phalen's Maneuver: The patient maintains maximal unforced wrist flexion (placing the dorsal surfaces of both hands together at 90 degrees) for 60 seconds. Reproduction or exacerbation of paresthesias in the median nerve digits constitutes a positive test.
  • Tinel's Sign at the Wrist: Light percussion with the examiner's fingertip over the volar surface of the flexor retinaculum reproduces tingling or electric-shock sensations radiating into the median digits.
  • Durkan's Carpal Compression Test: The examiner applies direct manual thumb pressure over the carpal tunnel for 30 seconds. This is the most sensitive provocative physical test for CTS.

2. Pronator Teres Syndrome

• Anatomy: Compression of the median nerve in the proximal volar forearm between the humeral and ulnar heads of the pronator teres muscle.
• Distinguishing Features from CTS:
  • Sensation over the thenar eminence of the palm is diminished (because the palmar cutaneous branch is compressed alongside the main median nerve trunk in the forearm).
  • Nocturnal awakening is generally absent.
  • Pain is centered in the proximal anterior forearm and is exacerbated by resisted active forearm pronation with elbow extension.

Ulnar Nerve: Cubital Tunnel Syndrome

• Anatomy: Entrapment of the ulnar nerve as it traverses the retroepicondylar groove behind the medial epicondyle of the humerus beneath Osborne's ligament (the cubital tunnel).
• Sensory Symptoms: Paresthesias, burning pain, and numbness along the ulnar aspect of the distal forearm, the entire little finger (5th digit), and the ulnar half of the ring finger (4th digit). Symptoms are exacerbated by prolonged elbow flexion (e.g., holding a telephone to the ear, sleeping with elbows tightly flexed).
• Motor Deficits & Clinical Signs:
  • Weakness and wasting of the hypothenar eminence and the first dorsal interosseous muscle (visible hollowing of the web space between the thumb and index finger on the dorsal hand).
  • Ulnar Claw Hand (Main en Griffe): In chronic denervation, the 4th and 5th digits develop hyperextension at the metacarpophalangeal (MCP) joints and flexion at the interphalangeal (IP) joints due to loss of the 3rd and 4th lumbricals.
  • Froment's Sign: Tests for motor paralysis of the adductor pollicis (innervated exclusively by the ulnar nerve). The patient is instructed to firmly grasp a flat sheet of paper between the thumb and the lateral index finger while the examiner attempts to pull it away. If the adductor pollicis is paralyzed, the patient involuntarily flexes the terminal interphalangeal (IP) joint of the thumb using the flexor pollicis longus (innervated by the median nerve via the anterior interosseous nerve) to maintain their grip. A positive Froment's sign confirms an ulnar nerve palsy.

Radial Nerve: Spiral Groove Compression vs. Posterior Interosseous Nerve

• Spiral Groove Compression ("Saturday Night Palsy" / "Honeymoon Palsy"):
  • Mechanism: Direct prolonged compressive trauma against the spiral groove of the mid-humeral shaft (classically occurring when an intoxicated individual falls asleep with an arm draped over the back of a rigid chair, or from improperly fitted axillary crutches).
  • Motor Deficits: Acute Wrist Drop—complete inability to actively extend the wrist and the metacarpophalangeal joints of the fingers and thumb (paralysis of extensor carpi radialis, extensor carpi ulnaris, extensor digitorum, and extensor pollicis longus). Triceps function is typically preserved if compression occurs at the mid-humerus, as branches to the triceps arise more proximally in the axilla.
  • Sensory Deficit: Numbness over the dorsal radial aspect of the hand, dorsal thumb, and dorsal first web space.
• Posterior Interosseous Nerve (PIN) Syndrome:
  • Compression of the deep motor branch of the radial nerve as it pierces the supinator muscle at the Arcade of Frohse.
  • Crucially, the PIN is a pure motor nerve; entrapment produces motor finger drop without any cutaneous sensory loss. In contrast, Radial Tunnel Syndrome produces severe aching pain 4–5 cm distal to the lateral epicondyle without overt motor weakness (frequently misdiagnosed as refractory lateral epicondylitis / tennis elbow).

Lateral Femoral Cutaneous Nerve: Meralgia Paresthetica

• Anatomy: The lateral femoral cutaneous nerve (LFCN, arising from lumbar nerve roots L2 and L3) courses through the pelvis and exits beneath or through the lateral attachment of the inguinal ligament, directly medial to the anterior superior iliac spine (ASIS).
• Etiology: Entrapment under the inguinal ligament triggered by obesity, rapid weight gain, pregnancy, tight low-rise jeans, or heavy utility belts (worn by police officers, carpenters, or military personnel).
• Clinical Presentation: Burning, stinging, tingling pain, hypersensitivity to clothing friction (hyperesthesia), or numbness localized strictly to the anterolateral aspect of the thigh.
• The Critical Clinical Boundary:
  • The lateral femoral cutaneous nerve is a purely sensory nerve.
  • In meralgia paresthetica, there is absolutely NO motor weakness of the quadriceps or iliopsoas, NO alteration of the patellar tendon reflex, and the pain never radiates below the knee. Finding motor weakness or reflex changes rules out meralgia paresthetica and points to an L2–L4 radiculopathy or femoral neuropathy.

Common Peroneal (Fibular) Nerve Entrapment

• Anatomy: The common fibular nerve branches from the sciatic nerve and winds superficial and subcutaneously around the neck of the fibula directly inferior to the lateral knee, making it the most frequently injured peripheral nerve in the lower extremity.
• Etiology: Direct external compression (prolonged crossed-leg sitting, tight casts, knee braces, pneumatic compression boots) or fibular head trauma/fracture.
• Motor Deficit: The nerve divides into deep and superficial branches. Damage paralyzes the anterior compartment muscles (tibialis anterior, extensor hallucis longus), causing profound Foot Drop (inability to dorsiflex the ankle or extend the toes). The patient exhibits a steppage gait (lifting the knee excessively high to avoid dragging the toes on the ground, followed by an audible foot slap). Paralysis of the superficial branch (peroneus longus and brevis) eliminates active foot eversion.
• Sensory Deficit: Numbness and paresthesias over the lateral lower leg and the entire dorsum of the foot.
• Differential Diagnosis: Common Fibular Palsy vs. L5 Radiculopathy:
  • Both produce foot drop and sensory loss over the dorsum of the foot.
  • The Differentiating Motor Sign: The common fibular nerve innervates muscles only distal to the knee. An L5 radiculopathy also denervates the Gluteus Medius (hip abductor) and Tibialis Posterior (foot inversion). If the patient demonstrates weak hip abduction (positive Trendelenburg sign) or weak foot inversion, the lesion is at the L5 nerve root, not the fibular head!

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Central vs. Peripheral Facial Palsy: The Critical Triage Distinction

Differentiating a peripheral seventh cranial nerve palsy (Bell's palsy) from an acute central cerebrovascular accident (CVA / stroke) is among the most heavily tested, life-critical diagnostic competencies on the NCCAOM Biomedicine module.

+-----------------------------------------------------------------------------+
|                   CENTRAL VS. PERIPHERAL FACIAL PALSY                       |
+-----------------------------------------------------------------------------+
|                                                                             |
|             [CENTRAL STROKE (UMN)]             [BELL'S PALSY (LMN)]         |
|            Corticobulbar Tract Lesion           Cranial Nerve VII Lesion    |
|                        |                                   |                |
|                        v                                   v                |
|              * FOREHEAD IS SPARED *             * ENTIRE HEMIFACE PARALYZED *
|          Patient CAN wrinkle forehead        CANNOT wrinkle forehead on side
|          Patient CAN tightly close eyes      CANNOT close eye (lagophthalmos)
|                        |                                   |                |
|                        v                                   v                |
|          Contralateral lower facial droop    Complete ipsilateral paralysis |
|          Flattened nasolabial fold           Bell's phenomenon (eye rolls up)
|          Drooping corner of mouth            Hyperacusis & loss of taste    |
|                        |                                   |                |
|                        v                                   v                |
|          [ACTIVATE 911 IMMEDIATELY]          [INITIATE MEDICAL / ACU TX]    |
|          Acute Stroke Reperfusion Window     Protect cornea with lubricant  |
|                                                                             |
+-----------------------------------------------------------------------------+

The Neuroanatomical Basis of Forehead Sparing

• The Facial Motor Nucleus resides in the lower pontine tegmentum. It is divided functionally into two sub-nuclei:
  1. The dorsal sub-nucleus innervates the upper facial muscles (the frontalis muscle which raises the eyebrow and wrinkles the forehead, and the upper fibers of orbicularis oculi which close the eye).
  2. The ventral sub-nucleus innervates the lower facial muscles (the orbicularis oris, buccinator, zygomaticus, and platysma which control smiling, whistling, and mouth closure).
• The Bilateral Innervation Axiom:
  • The motor neurons supplying the upper face receive BILATERAL corticobulbar input—they receive descending upper motor neuron fibers from BOTH the left and right cerebral motor cortices.
  • The motor neurons supplying the lower face receive ONLY CONTRALATERAL corticobulbar input—they receive fibers exclusively from the opposite cerebral cortex.

   Left Motor Cortex              Right Motor Cortex
         \    /                        \    /
          \  /                          \  /
           \/                            \/
   [Bilateral Cortical Input]    [Bilateral Cortical Input]
            |                                 |
            v                                 v
   Upper Face Motor Nucleus       Upper Face Motor Nucleus
   (Left Pontine Nucleus)         (Right Pontine Nucleus)
            |                                 |
            v                                 v
   [Left Frontalis Muscle]        [Right Frontalis Muscle]
   *If one cortex has a stroke,   *The opposite intact cortex
    the other cortex still sends   still provides full motor
    signals to BOTH sides!*        innervation to BOTH sides!*

Clinical Presentation Differences

1. Acute Central Stroke (Upper Motor Neuron [UMN] Lesion):

   • Lesion Site: Contralateral cerebral motor cortex, internal capsule, or upper brainstem.
   • Physical Presentation: FOREHEAD SPARING. When instructed to look up at the ceiling or wrinkle their forehead, the patient wrinkles both sides of their forehead symmetrically and effortlessly. When asked to shut their eyes tightly, they can close both eyes with equal power. However, the contralateral lower face is paralyzed—the corner of the mouth droops, the nasolabial fold is completely flattened, and saliva drools from the affected side.
   • Associated Findings: Frequently accompanied by other central neurological deficits, such as ipsilateral hemiparesis (arm/leg drift), aphasia, dysarthria, or sensory extinction.
   • Triage Action: Call 911 immediately. This is an acute ischemic stroke or intracranial hemorrhage. The therapeutic window for intravenous thrombolysis (alteplase / tenecteplase) is <= 4.5 hours from last known normal, and endovascular mechanical thrombectomy is up to 24 hours for large vessel occlusions.

2. Bell's Palsy (Lower Motor Neuron [LMN] Lesion - Cranial Nerve VII):

   • Lesion Site: The facial motor nucleus in the pons, or the peripheral trunk of cranial nerve VII (often triggered by herpes simplex virus type 1 reactivation and swelling within the narrow fallopian canal).
   • Physical Presentation: COMPLETE IPSILATERAL FACIAL PARALYSIS. All peripheral motor fibers supplying that half of the face are interrupted. The patient:
     • CANNOT wrinkle the forehead on the affected side (the forehead remains completely smooth and unwrinkled when looking up).
     • CANNOT close the eye completely on the affected side (lagophthalmos).
     • Bell's Phenomenon: When the patient attempts to close their eyes, the paralyzed eyelid remains open while the eyeball visibly rolls upward and outward to protect the cornea.
     • Drooping mouth, flattened nasolabial fold, inability to purse the lips, whistle, or puff the cheeks.
   • Associated Symptoms: Hyperacusis (painful sensitivity to loud sounds caused by paralysis of the stapedius muscle in the middle ear), loss of taste sensation on the anterior two-thirds of the tongue (involvement of the chorda tympani), and retroauricular pain.
   • Clinical Management: Urgent outpatient referral to a physician within 72 hours for high-dose oral corticosteroids (prednisone 60 mg daily for 7 days) and antiviral therapy (valacyclovir). Acupuncturists must immediately instruct the patient on corneal protection: daytime lubricating eye drops, nocturnal ophthalmic ointment, and taping the eyelid closed at night to prevent exposure keratitis, corneal drying, and permanent blindness.